This invention relates generally to an internal combustion engine capable of operating in a premixed charge compression ignition mode while effectively controlling the start of combustion.
For well over 75 years the internal combustion engine has been mankind""s primary source of motive power. It would be difficult to overstate its importance or the engineering effort expended in seeking its perfection. So mature and well understood is the art of internal combustion engine design that most so called xe2x80x9cnewxe2x80x9d engine designs are merely designs made up of choices among a variety of known alternatives. For example, an improved output torque curve can easily be achieved by sacrificing engine fuel economy. Emissions abatement or improved reliability can also be achieved with an increase in cost. Still other objectives can be achieved such as increased power and reduced size and/or weight but normally at a sacrifice of both fuel efficiency and low cost.
The challenge to contemporary designers has been significantly increased by the need to respond to governmentally mandated emissions abatement standards while maintaining or improving fuel efficiency. In view of the mature nature of engine design, it is extremely difficult to extract both improved engine performance and emissions abatement from further innovations of the basic engine designs commercially available today. Yet the need for such innovations has never been greater in view of the series of escalating emissions standards mandated for the future by the United States government and other countries. Attempts to meet these standards include some designers looking for a completely new engine design.
Traditionally, there have been two primary forms of reciprocating piston or rotary internal combustion engines: diesel and spark ignition engines. While these engine types have similar architecture and mechanical workings, each has distinct operating properties which are vastly different from each other. Diesel and spark ignited engines effectively control the start of combustion (SOC) using simple, yet distinct means. The diesel engine controls the SOC by the timing of fuel injection. In a spark ignited engine, the SOC is controlled by the spark timing. As a result, there are important differences in the advantages and disadvantages of diesel and spark-ignited engines. The major advantage that a spark-ignited natural gas, or gasoline, engine has over a diesel engine is the ability to achieve extremely low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines (such as passenger car gasoline engines and lean burn natural gas engines) is higher thermal efficiency. One key reason for the higher efficiency of diesel engines is the ability to use higher compression ratios than premixed charge spark ignited engines (the compression ratio in premixed charge spark ignited engines has to be kept relatively low to avoid knock). A second key reason for the higher efficiency of diesel engines lies in the ability to control the diesel engine""s power output without a throttle. This eliminates the throttling losses of premixed charge spark ignited engines and results in significantly higher efficiency at part load for diesel engines. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels which are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion a large fraction of the fuel exists at a very fuel rich equivalence ratio which is known to lead to particulate emissions. Premixed charge spark ignited engines, on the other hand, have nearly homogeneous air fuel mixtures which tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. Another consideration is that the mixing controlled combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause high NOx emissions. Lean burn premixed charge spark ignited engines, on the other hand, burn their fuel at much leaner equivalence ratios which results in significantly lower temperatures leading to much lower NOx emissions. Stoichiometric premixed charge spark ignited engines, on the other hand, have high NOx emissions due to the high flame temperatures resulting from stoichiometric combustion. However, the virtually oxygen free exhaust allows the NOx emissions to be reduced to very low levels with a three-way catalyst.
Relatively recently, some engine designers have directed their efforts to another type of engine which utilizes premixed charge compression ignition (PCCI) or homogeneous charge compression ignition (HCCI), hereinafter collectively referred to as PCCI. Engines operating on PCCI principles rely on autoignition of a relatively well premixed fuel/air mixture to initiate combustion. Importantly, the fuel and air are mixed upstream of the cylinder, e.g., in the intake port, or in the cylinder, long before ignition occurs. The extent of the mixture may be varied depending on the combustion characteristics desired. Some engines are designed and/or operated to ensure the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Also, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. Thus, PCCI combustion is characterized in that: 1) the vast majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge by the time of ignition; and 2) ignition, that is, the very onset or start of combustion, is initiated by compression ignition. Unlike a diesel engine, the timing of the fuel delivery, for example the timing of injection, in a PCCI engine does not strongly affect the timing of ignition. Preferably, PCCI combustion is characterized in that most of the mixture is significantly leaner than stoichiometric to advantageously reduce emissions, unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion.
Because an engine operating on PCCI combustion principles has the potential for providing the excellent fuel economy of the diesel engine while providing NOx and particulate emissions levels that are much lower than that of current spark-ignited engine, it has also recently been the subject of extensive research and development. U.S. Pat. Nos. 4,768,481; 5,535,716; and 5,832,880 all disclose engines and methods for controlling PCCI combustion in engines. Researchers have used various other names in referencing PCCI combustion including homogeneous charge compression ignition (HCCI) as well as others such as xe2x80x9cATACxe2x80x9d which stands for xe2x80x9cActive Thermo-Atmosphere Combustion.xe2x80x9d (SAE Technical Paper No. 790501, Feb. 26-Mar. 2, 1979), xe2x80x9cTSxe2x80x9d which stands for xe2x80x9cToyota-Sokenxe2x80x9d (SAE Technical Paper No. 790840, Sep. 10-13, 1979), and xe2x80x9cCIHCxe2x80x9d which stands for xe2x80x9ccompression-ignited homogeneous chargexe2x80x9d (SAE Paper No. 830264, 1983). All of these terms are hereinafter collectively referred to as PCCI.
Although PCCI combustion may result in improved fuel economy and substantially reduced emissions, it is difficult for an engine to operate in a PCCI mode over a wide range of operating conditions, ranging from cold start-up to various levels of engine load. For example, SAE Technical Paper No. 790501 reports that PCCI combustion (ATAC) could be made to occur in a two-stroke engine at low load over a wide speed range. To attain PCCI combustion, the following conditions were found to be important. The quantity of mixture and the air/fuel ratio supplied to the cylinder must be uniform from cycle to cycle. The scavenging xe2x80x9cdirectivityxe2x80x9d and velocity must have cyclic regularity to ensure the correct condition of the residual gases remaining in the cylinder. The temperature of the combustion chamber walls must be suitable. The scavenging passage inlet must be located at the bottom of the crankcase. It was found that at very light loads, PCCI was not successful because charge temperatures were too low. At very high loads, PCCI was not successful because the residual gas quantity was too low. In between these regions, PCCI combustion was successful.
As a result, research has been directed to an engine capable of operating in multiple combustion modes. For example, SAE Technical Paper No. 892068, entitled xe2x80x9cHomogeneous-Charge Compression Ignition (HCCI) Enginesxe2x80x9d, Thring, R., Sep. 25, 1989, investigated PCCI operation of a four-stroke engine. The paper suggests an engine that would operate in a conventional spark-ignition mode at start-up and at high loads, but in a PCCI mode at part-load and idle. Others have produced two-stroke motorcycle engines which successfully use a spark to initiate combustion upon starting the engine, at the lowest load conditions, such as idling, and at high loads while operating in a PCCI mode during a low to mid-load range. SAE papers 920512 and 972874 are noted for disclosing experimental results comparing PCCI combustion to spark-ignition combustion. German Patent No. 198 18 596 also discloses a process of operating an engine in a PCCI mode at least low loads and in a spark-ignition mode at high loads.
Given the benefits of reduced emissions and improved fuel economy when operating in the PCCI mode, others have focused on practical solutions for stabilizing PCCI operation throughout changing engine conditions. Patent application Ser. No. 09/255,780 filed on Feb. 23, 1999 (published as International Patent Application No. PCT/US99/03289), currently assigned to the Assignee of the present invention, discloses an engine and method of operation which includes various control features for more effectively controlling the start of combustion. The application recognizes the ability to control in-cylinder temperature by controlling IMT thereby effectively controlling the start and/or duration of combustion. Moreover, the application recognizes that it is possible to advance the combustion event by reducing the engine speed, and to retard the combustion event by increasing the engine speed.
Still, there is a need for an engine, and method of engine operation, which includes more effectively and more efficiently operating in a PCCI mode, including simple, effective control of the start of combustion.
A general object of the subject invention is to overcome the deficiencies of the prior art and to provide a practical engine and a method for operating the engine in a premixed charge compression ignition mode while minimizing the need to vary IMT.
Another object of the present invention is to provide a PCCI engine capable of controlling SOC throughout engine load changing conditions.
Yet another object of the present invention is to provide a PCCI engine capable of more rapidly and smoothly controlling SOC.
Still another object of the present invention is to provide a PCCI engine capable of operating over a wider range of loads without the need to adjust IMT to undesirable levels or levels difficult to achieve.
A further object of the present invention is to provide a PCCI engine capable of achieving more efficient operation in hybrid and power generations applications.
A further object of the present invention is to provide a PCCI engine capable of achieving more efficient operation in conjunction with a continuously variable transmission.
A further object of the present invention is to provide a PCCI engine capable of operating on a single fuel throughout operation.
A still further object of the present invention is to provide a PCCI engine capable of minimizing emissions and avoiding very heavy, destructive knock and misfire.
The above objects and others are achieved by providing an internal combustion engine operable in a premixed charge compression ignition mode at an engine speed and an engine torque corresponding to an engine horsepower output, comprising an engine body, a combustion chamber formed in the engine body, an intake air system for delivering intake air to the combustion chamber, and a fuel delivery system mounted on the engine body to delivery to at least one of the intake air system and the combustion chamber, wherein the supply fuel and the intake air forms a premixed charge. The engine further includes a control system adapted to adjust the engine torque and adjust the engine speed when the engine is operating in the premixed charge compression ignition mode to vary a timing of a start of combustion of the premixed charge while delivering a targeted, e.g. requested or desired, engine horsepower output.
The engine may further include a combustion sensor connected to control system for sensing the start of combustion and generating a start of combustion signal, wherein the control system adapted to control a start of combustion based on the start of combustion signal. The engine may further include a turbocharger. The fuel delivery system may supply only a single type of fuel to the engine or more than one type of fuel. The control system may be further adapted to cause the fuel delivery system to deliver a post-ignition injection of fuel into the combustion chamber after the start of combustion of the premixed charge in the combustion chamber when in the premixed charge compression ignition mode. The fuel may be one of diesel fuel, kerosene, gasoline, natural gas, hydrogen and propane. The engine may further include an engine torque control system controlled by the control system to increase the engine torque to advance the start of combustion. The control system may be adapted to control the fuel delivery system to adjust a quantity of the fuel delivered to one of the intake air system and the combustion chamber.
The present invention is also directed to a method of operating an internal combustion engine in a premixed charge compression ignition mode at an engine speed and an engine torque corresponding to an engine horsepower output, comprising the steps of providing intake air to a combustion chamber and delivering fuel to at least one of the intake air system and the combustion chamber, where the fuel and the intake air form a premixed charge. The method further includes adjusting the engine speed and adjusting the engine torque when the engine is operating in the premixed charge compression ignition mode to vary a timing of a start of combustion of the premixed charge while delivering a targeted, e.g., requested or desired, engine horsepower output.